In integrated circuit interconnection structures, a stack of layers are assembled in which the individual metal layers provide different functions. There are situations where efforts are required to prevent unwanted interaction between layers. There is such a situation in connection with the use of lead free solder and in particular to the prevention of the formation of intermetallic compound inclusions that form during reflow connecting where electroless Ni(P) metallization is present.
Electroless nickel-phosphorus (Ni—P) films are widely used in the microelectronic industry for several types of metallizations. They have such characteristics as excellent solderability, corrosion resistance, uniform thickness and selective deposition.
The electroless nickel phosphorous technology and its applications is well known and described in such publications as: Wiegele et al, in the Proceedings of IEEE Electronic Component and Technology Conference, 1998, p. 861; Mei et al in the Proceedings of the IEEE Electronic Component and Technology Conference, 1998, p. 952; Lin et al in the Proceedings of IEEE Electronic Component and Technology Conference, 2001, p. 455; K. C. Hung in the Proceedings of IEEE Electronic Component and Technology Conference, 2002, p. 1650; and O. Villalobos in the Proceedings of IEEE Electronic Component and Technology Conference, 2002, p. 732.
In this technology, when an Ni—P film reacts with Sn—Pb eutectic solder, a part of the film underneath the solder crystallizes into Ni3P with a (P-rich layer); that forms at about the reflow temperature of about 200˜240° C. This low temperature reaction is referred to in the art as “solder reaction-assisted crystallization” and is described in a publication by Kim et al in the J Appl. Phys 85, 8456(1999). The “solder reaction-assisted crystallization” is different from the well known self-crystallization of Ni—P that occurs at a higher temperature.
The solder reaction-assisted crystallization is accompanied by the formation of inclusions of Ni—Sn intermetallic compounds and the formation of voids in the layer known as Kirkendall voids as described by Hung et al in the Mater. Res. publication Vol. 15, pg 2534, (2000); and by P. L. Liu et al in the Metall. Mater. Trans. publication Vol. A 31A, pg 2857, (2000).
Such interfacial reactions affect reliability and are often attributed as being the source of formation of a weak and brittle interface between Ni—P and Sn—Pb solder, as described in publications by; R W Wiegele et al. in the Proceedings of IEEE Electronic Component and Technology Conference, 1998, p. 861; by Mei et al in the Proceedings of the IEEE Electronic Component and Technology Conference, 1998, p. 9520 and by Villalobos in the Proceedings of IEEE Electronic Component and Technology Conference, 2002, p. 732.
When Sn—Pb solder is replaced by Sn-rich Pb-free solders, the reliability issue of the Ni—P interface is expected to be even more important since Pb-free solders have a higher
Sn content and a higher reflow temperature as described by K. C. Hung in the Proceedings of the IEEE Electronic Component and Technology Conference, 2002, p. 1650; and by. K. Zeng et al. in Materials Science and Engineering R 38, 55 (2002).
Electroless Ni(P) is a good candidate as a reaction barrier for Pb-free, Sn-rich solders, because the intermetallic compounds forming on an electroless Ni(P) surface tend to grow more slowly than on Cu metallization during soldering.
However, severe inclusions or spalling of intermetallic compounds from Ni(P) have been reported by Kang et al in the Proceedings of the 51st ECTC May 2001 pgs 448-454; when P-free solders such as pure Sn, Sn-3.5Ag, Sn-3.5Ag-3Bi (in weight %) are applied in a form of solder paste onto an electroless Ni(P) layer. Typical examples of intermetallic compounds inclusions occur where for example Sn-3.5Ag solder paste is applied on an Ni(P) layer and reflowed at 250 C for durations of between 2 min and 10 min. A further example of intermetallic compound spalling occurs when Sn solder is electroplated onto Ni(P) and the reflow condition is severe such as for 10 min reflow at 250 C or the Ni(P) is electroless Ni(P).
The delamination or spalling of intermetallic compounds at the soldering interface is a reliability risk factor in thermo-mechanical solder joints.
There have been earlier efforts involving such metals as Au,Ag and Pd. In those efforts a thin layer of Au on top of Ni(P) metallization did not protect the Ni(P) and therefore intermetallic compound formation or spalling was observed. In the case of an Au layer, the dissolution rate of Au into the molten Sn-rich solder, such as Sn-3.5% Ag, is expected to be so rapid that it can not protect the Ni(P) metallization. A similar situation would be expected with a thin layer of Ag or Pd on top of Ni(P) metallization.